Speech-sign bilingualism is exceptional because distinct sensory-motoric modalities allow for the simultaneous production and perception of two languages. Whereas unimodal (speech-speech) bilinguals may code-switch between their two languages, bimodal (speech-sign) bilinguals produce code-blends (simultaneous sign and speech). We investigate the consequences of the sensory-motoric differences in language modality for the psycholinguistics of bilingualism, for the features of co-speech gesture, and for the nature of the bilingual brain. Hearing ASL-English bilinguals will be studied because both languages are naturally accessible in the environment for these individuals. We proceed along three lines of inquiry: I. The psycholinguistics of bimodal bilingualism: Using behavioral tasks, we investigate the architecture of the bilingual lexicon across two modalities, the levels at which a bimodal bilinguars languages are interconnected, and how bimodal bilinguals control their two language systems. We hypothesize that, in general, bimodal and unimodal bilinguals are quite similar, but unique effects of bimodal bilingualism will be found in the recognition and production of code-blends. II. The relation between bimodal bilingualism and co-speech gesture: Unlike code-switching, code-blending shares certain properties with co-speech gesture (the spontaneous gestures that accompany spoken language). For both, meaningful manual gestures are produced simultaneously with speech. We explore 1) parallels and differences between bimodal code blending and unimodal code-switching, 2) parallels and differences between code-blending and co-speech gesture, and 3) the impact of acquiring a signed language on co-speech gesture. III. The cognitive neuropsychology of bimodal bilingualism: Using functional imaging, we address the following questions: What patterns of overlapping and distinct activation are observed for ASL and spoken English at the lexical and sentential levels? How does age of ASL acquisition impact the neural systems recruited during sign comprehension? Does acquisition of two spoken languages early in development differ from early acquisition of a signed and a spoken language with respect to the neural organization of language systems? Does becoming a bimodal bilingual impact the neural organization for non-linguistic visual processing? Answers to these questions will not only inform us about the development of the bilingual brain but will also provide insight into the determinants of neural plasticity and the hemispheric specialization for language.